This invention relates to a process and an apparatus for producing hydrogen for application to fuel cell electric generators.
Catalytic reaction apparatus and processes for converting hydrocarbon feedstocks to useful industrial gases, such as hydrogen, is well known in the art. Proton exchange membrane (PEM) fuel cells have emerged as a viable option for the production of disbursed electrical power, typically in the range of 2–50 k W, for use in residential and small commercial applications. PEM fuel cells generate electricity by the electrochemical reaction between hydrogen and oxygen.
While oxygen is readily available from ambient air, hydrogen must be produced from commercially available fuels, such a natural gas or propane, using methods such as steam reforming. Steam reforming is a process that involves a high temperature catalytic reaction between a hydrocarbon and steam to form a hydrogen-rich product gas, commonly referred to as reformate, that contains significant quantities of carbon monoxide.
Since PEM fuel cells have a low tolerance to carbon monoxide, the concentration of carbon monoxide in the reformate must be reduced using a catalytic reaction step known as the water-gas shift reaction. Following the water-gas shift reaction, the concentration of carbon monoxide in the reformate is further reduced to concentrations typically less than 10 ppm using a selective oxidation reaction, also referred to as preferential oxidation or PROX. The combination of processes that convert commercial fuels to a reformate suitable for use in a fuel cell is commonly referred to as a fuel processor.
As an illustration, Table 1 summarizes the reaction steps of a fuel processor designed to produce a hydrogen-rich gas stream suitable for use in a PEM fuel cell.
TABLE 1PEM fuel processor reaction steps1. CH4 + H2O = CO + H2Steam reforming2. CO + H2O = CO2 + H2Water-gas shift3. CO + ½O2 = CO2Selective oxidation
In typical industrial practice, the water-gas shift reaction is conducted in two separate adiabatic reactors operating at two different temperature regimes. The first reactor, known as the high temperature shift reactor, operates at inlet temperatures typically ranging from about 550° F. to 650° F. The second reactor, known as the low temperature shift reactor, operates at an inlet temperature typically ranging from about 380° F. to 450° F. The combination of the two sequential water-gas shift reactions typically reduces the concentration of carbon monoxide in the reformate to less than 1.0 volume percent.
The use of a low temperature shift reactor is beneficial because the water-gas shift reaction is thermodynamically favored at lower temperatures. However, a high temperature shift reactor is generally required to limit the amount of heat that is released in the low temperature shift reactor.
Conventional low temperature shift catalysts comprise a mixture of copper and zinc that are supported on a ceramic carrier. These catalysts promote the water-gas shift reaction at lower temperature, but lose activity if they are exposed to excessively high temperatures due to sintering of the active metals. Therefore, it is generally desirable to limit the maximum temperature of the low temperature shift catalyst to about 500° F. in order to achieve long catalyst life.
The water-gas shift reaction releases approximately 9837 calories per gram-mole of carbon monoxide that is consumed. If the water-gas shift reaction were conducted using a single adiabatic low temperature shift reactor, the heat release would result in a temperature increase across the catalyst bed that would exceed the desirable temperature limit for conventional low temperature shift reactors. Furthermore, the high exit temperature from the water-gas shift reactor would be thermodynamically less favorable for achieving high conversions of carbon monoxide.
There is need to minimize the number of reactors and heat exchangers that are needed to achieve the objective of high conversion of carbon monoxide for PEM fuel cell applications, in order to reduce the size, cost and complexity of the fuel processor. Therefore, it is desirable to conduct the water-gas shift reaction using a single reactor vessel that is maintained within acceptable operating temperature limits by controlling heat removal from the reactor.
The steam reforming reaction requires large quantities of steam for the conversion of hydrocarbon to reformate. It is desirable to recover the heat released from the water-gas shift reaction for the purpose of generating steam in order to improve the thermal efficiency of the fuel processor. The present invention achieves the objective of temperature control and heat recovery by integrating a lower temperature shift reactor within a steam generator that contains water boiling at a temperature range of about 360° F. to 400° F., corresponding to a boiler pressure of about 153 psia to 247 psia.
Because the vessel walls of the lower temperature shift reactor are in heat transfer communication with boiling water, the heat released from the water-gas shift reaction is effectively removed to control the temperature in the catalyst bed within the desired operating temperature range. Furthermore, the heat released from the water-gas shift reaction is beneficially recovered to generate steam that is used in the process. Finally, the steam generator provides a convenient source of heat for heating the catalyst bed during start-up.
U.S. Pat. No. 6,086,840 describes a process for making ammonia that mentions use of an isothermal shift reactor that includes heat exchange tubes extending within a vessel packed with catalyst. The heat exchange tubes contain a boiling fluid to remove heat from the catalyst bed.